A method of removing water from an aircraft fuel tank is described in U.S. Pat. No. 4,809,934. A water scavenge pipe collects water which is dispersed in the fuel immediately upstream of a booster pump. A first problem with this system is that it requires a complex arrangement of venturis and jet pumps. A further problem is that water may be fed to the engine when the engine is operating at a relatively low rate. This may reduce the operating efficiency of the engine. A further problem is that the system may not remove water entirely from the fuel tank during a single flight.
A method of removing water from a fuel tank is described in U.S. Pat. No. 6,170,470. A water line feeds water into an engine from the fuel tank. A valve in a water line is opened when the speed of the engine is above a minimum threshold, because water flowing into the engine when the engine is operating at relatively high speeds will not have a significantly adverse effect on the operational characteristics of the engine. A problem with the method of U.S. Pat. No. 6,170,470 is that an engine speed measuring device must be provided, along with a communication line to the valve. Both of these elements are at risk of failing, and the communication line must be routed out of the fuel tank in a fluid-tight and inherently safe manner.
A particular difficulty associated with an aircraft fuel system is that once the aircraft has taken off the ambient air temperature drops to −30 or −40 deg C. and the water in the fuel freezes after approximately 20 minutes. After the water has frozen it is trapped in the tank until the tank thaws on landing. Therefore if the water is not removed in the first twenty minutes it will gradually accumulate over several flights until it causes problems or is manually drained.